OBSERVATIONS ON STRIPED AND tJNSTRIPED MUSCLE. 93 



These considerations show that the function of intracellular 

 networks is very probably of an active nature. 



We have now to consider the networks of striped and unstriped 

 muscle. Both these forms of network are non-essential to contraction, 

 for we have seen that many muscle-fibres of invertebrates are devoid 

 of a network of any kind ; but that they modify the nature of the 

 contraction is very probable. 



We have seen that the network of striped muscle is developed 

 when rapid movements are to be performed ; this shows that the 

 function of contraction is intimately associated with the presence of 

 the network. 



The chief points of difference in the contraction of striped and 

 unstriped muscle respectively are the great length of the latent 

 period and the long duration of the contraction in the unstriped 

 muscle. The velocity of the contraction-wave in striped muscle is 

 in the Frog, 3 — 4 metres per second, while in the unstriped muscle 

 (ureter) the velocity is only 20 — 30 mm. per second.* This seems 

 to indicate that the peculiarly arranged network of striped muscle 

 may be associated with the rapidity of its contraction. 



In nearly all the specimens I have examined, both the transverse and 

 longitudinal bars of the network remain perfectly straight in all conditions 

 of contraction and relaxation of the muscle. Hence the network, or part of 

 the network, must either contract to the full extent that the muscle-fibre 

 does, or else be elastic, and so follow the movements of contraction of the fibre. 



Eetziusf figures a specimen in which the longitudinal bars are 

 zigzag. However, from his description, and from comparison with 

 my own preparations, I believe this to be due to disturbance during 

 the preparation, and not to be a normal condition. 



We have now to consider whether the network is actively con- 

 tractile or merely a passively elastic structure ; or whether one part 

 of it is contractile and the other passive. That both network and 

 sarcous substance are contractile is improbable ; for if the function 

 of the network and the sarcous substance is identical, there is no 

 apparent reason for the presence of the network. Differentiation in 

 structure always implies differentiation in function. 



* " Text-book of Physiology," Dr. Michael Foster, 4th cd., p. 101. 

 t Loc. cit., pi. i., Fig. 19. 



